A Certain Butterfly Effect
by clockworkBabbage
Summary: If Index never fell onto Touma' balcony, a certain unlucky youth - and everyone around him - would have a far different life.
1. Prologue

Academy city.

The most technologically advanced city on the planet. Some estimates place it at least 20 to 30 years ahead of the rest of the world in terms of scientific development and research. This is due in part, at least, to the sheer amount of schools, universities, and research institutions in the city. After all, 80% of its 2.3 million inhabitants are students. They aren't exactly normal students, though. Most are involved in Academy City's Power Curriculum Program: a special program designed to create individuals with tremendous psychic powers. These individuals - espers - possess abilities ranging from basic telekinesis to exotic ones such as teleportation, invisibility, or control over specific substances or energies.

It goes without saying that the things which happen in such a city are often far from normal. In fact, it would probably be fair to say that it would be strange if bizarre occurrences didn't happen every day. The small, silver-haired girl currently fleeing across the dark rooftops of the city seems, honestly, pretty mundane by comparison.

* * *

><p>She'd been running for what seemed like forever. Really, it was more like only a few minutes, but sprinting across rooftops at top speed while leaping between buildings isn't exactly easy.<p>

She certainly didn't look athletic, and her curious attire wasn't helping her running. She was dressed in what appeared to be a nun's habit, except instead of the traditional black and white robes, hers was white with gold trimming. It looked like possibly the worst thing one could be wearing while trying to run - in fact, the girl had already stumbled over it several times. At first glance, it was hard to understand how her pursuers hadn't caught up to her yet.

Then again, the girl wasn't simply trying to outrun them. She wouldn't be bothering to do something as dangerous as running across the skyline of the city if she could get away through other means. Despite appearances, the girl had a lot of practice with evading capture. She'd been fleeing from various pursuers for almost as long as she could remember, and she knew that she had to rely on cunning and trickery to slow them down or to make it more difficult for them to follow her. Taking to the rooftops was just one of these tricks.

Although she'd been on the run for basically her entire life, that life was only about a year long. Her memory, which otherwise was exceptionally perfect, had no recollection of anything that happened before July 28th of the previous year. For all intents and purposes, she had been born on that day. Everything before then was blank - there was absolutely nothing there. Well, nothing except for the library of books stored in her memory.

Those books - 103,000 of them - were the entire reason she was being chased. She could not allow herself - or, more accurately, the invaluable contents of her memories - to be captured. She would utilize every trick she knew to avoid her pursuers and to prevent her knowledge from falling into enemy hands.

The girl leaped onto a neighboring rooftop and quickly glanced around, still running, in order to figure out her next move. Her eyes widened slightly in panic as she surveyed the surrounding rooftops: she had nowhere left to run. There were no other buildings that she could safely jump to, as the closest rooftop she could find was much farther away than any of the others she had been able to manage. But she couldn't hesitate or turn back, as that would only lead to her capture. She had no choice but to take a leap of faith and trust in God to help her across.

Summoning up the reserves for an extra burst of speed, she raced towards the edge of the roof, tensed, and leaped outwards while giving a silent mental prayer.

Her best effort wasn't enough, though. The gap between the buildings was simply too large, and time seemed to slow for her as she fell through empty space. But she couldn't allow herself to just give up yet. Protecting the library stored inside her head was her duty, and she would not allow herself to fail at it. The girl desperately stretched out her arms towards the rooftop, almost as if willing it closer to her...

In another universe, she didn't make it. In another universe, she crashed down into the railing of a balcony a few floors below and passed out. In another universe, the life of one boy was changed forever. But here, in this universe...

The girl gasped as her body slammed into the opposite wall, the impact driving the air out of her lungs. But, she had managed - just barely - to latch onto the roof with her fingertips the moment before. The girl miraculously was able to hang on, thankful for the fact that her strange robes had stopped her from experiencing any pain or actual injury from the collision. Her feet scrabbled on the building, searching frantically for any kind of purchase. They found the top edge of a window sill to stand precariously on, and she was able to quickly shift her hands to a better position on the ledge above. Now that she had a better grip, she started to pull herself up. Her shoes were able to gain a small amount of traction on the rough concrete walls, and she managed to climb, gasping, onto the roof.

She rolled over onto her back and lay there for a minute, panting heavily and trying to calm her raging heartbeat. A lightning bolt flashed across her vision, and she wondered for a moment at what could possibly have caused it when there wasn't a cloud in the sky. Realizing that she'd wasted too much of her time lying there already, the girl shakily forced herself to her feet and, after a short prayer of thanks, disappeared into the night.

* * *

><p>A few hours later, a teenage boy with black spiky hair trudged exhaustedly up the stairs of the building the girl had almost fallen from. He stopped in front of a door with a nameplate bearing the name 'Kamijou Touma,' and began to fumble for his keys. "I mean, what the hell is that crazy Biri-Biri's problem?" he muttered exasperatedly to himself as he unlocked the door to his room. He dazedly flicked the light switch on as he shuffled into his room, and stared dumbly as light completely failed to illuminate his dorm. He toggled the switch a few more times in disbelief before dejectedly giving up.<p>

"Such misfortune!" he groaned, hanging his head. "Don't tell me she blew out the city's power grid with her damn lightning. It's almost like she's actively trying to make my life hell." The boy let his bag slide to the floor, and attempted to gingerly navigate his way through the dark room. Despite his almost excessive caution, he still somehow managed to trip several times more than anyone else could have managed. He finally reached his bed and let himself fall into it, grumbling all the while. He was asleep before he hit the pillow, oblivious to how close he'd come to getting drawn into an incomprehensible world.

* * *

><p><strong>Author's Note:<strong> So, I've had this idea running through my head for quite a while now, and I figured it was about time I try and get it down on paper. Basically, this story is my answer to what would happen if Index never fell into Touma's life, but all the various plots and plans in the series continued on regardless. This is not to say she - and the rest of the magic side - won't be making an appearance here. Far from it. It's just that the two worlds will remain more separate than they did before, at least for awhile.

I don't claim to be any kind of great writer, and this is also the first time I'm really doing anything like this. Therefore, any and all sorts of constructive criticism would be greatly appreciated.

Also, I'm going to have to apologize in advance for what will likely be sporadic updates, due to a number of things (the big ones being school and the fact that I'm rather lazy). I'll try my best to make sure I keep the story going, though.

Anyway, I hope you like how this turns out. Thanks for reading!


	2. A Not So Ordinary Day

"You know, it's bad enough having an alarm clock to worry about without nature itself getting in the way of my sleep," moaned Kamijou Touma as he lay sprawled lazily on his bed. Not only was the sun shining right into his face, but it was uncomfortably hot. Today was July 20th, the first day of summer vacation, too. The fact that the world itself didn't seem to want him to be able to sleep in just wasn't fair.

"...wait a minute, alarm clock?" The boy sat bolt upright, now wide awake, and turned to look at the clock by his bed. Instead of displaying the time normally, the numbers on the clock were flashing and were stuck on 00:00.

"Uggh…" he groaned, remembering the power outage caused by a certain electrifying middle-school student whose name he'd never bother to learn. This would, he guessed, also explain why the A/C didn't seem to be working. "Dammit, now I'm going to be late."

He rolled out of bed and opened the refrigerator door, hoping to find something quick to eat before heading off to school. Today might be the first day of summer vacation, but Touma had missed so much class during the year that he'd been assigned remedial lessons. Unfortunately, the power outage had affected the refrigerator as well. The stench of spoiled food made him shut the door almost immediately.

"Well, if all the food's spoiled, I'll just have to go buy some more. Now where's my-"

A sharp cracking sound coming from under his foot interrupted him. Realizing that he'd stepped on his wallet, he gingerly moved his foot away and picked the wallet up. Opening it, he discovered that his ATM card – his main source of money - had snapped.

"Such misfortune!" he groaned. He'd have to replace the card before he could go buy anything now - a simple enough matter, but with him being late for school anyway it effectively ruined any chances he had of being able to eat.

He changed into his school uniform and rushed out the door, stomach growling loudly all the while. It looked like today was going to be just another normal, unlucky day.

* * *

><p>"Only ¥30 left, huh?" Touma sighed as he looked morosely through his barren wallet. It was early evening, and Touma had just finished eating his first and only meal of the day. Well, if you could even call it a meal. For a teenage boy, the meager amount of food he'd been able to afford was more like a snack than anything else.<p>

In addition to the food issue, his day had been rather horrible even by his low standards. First of all, rather than being late, he actually was a full hour early for class. Had his clock been working, he'd have realized that he would have had the time to get his ATM card replaced and wouldn't have had to go hungry all day. Then, as punishment for not paying attention in class, he'd been forced to stay behind through normal school hours trying to perform Esper tests that were simply impossible for Level 0's like him. And as if that wasn't enough, that Biri-Biri girl had tried to fight him yet _again_ on his way home. He'd somehow managed to talk her out of it, but he seriously doubted he'd ever be that lucky again.

"Well, at least today can only get better from here," he said to himself with a note of forced optimism as he walked down the empty streets. It had cooled down considerably from the midday heat, and there was a pleasant breeze blowing. Touma was actually almost glad he'd missed the bus; he'd always enjoyed taking walks, even to the point where he'd sometimes break curfew late at night just to walk through the city.

Touma suddenly stopped in his tracks. Something in the back of his mind had been bugging him for several minutes, and he'd only now realized what it was. "It's far too early for curfew. So where is everyone?" he wondered aloud. It wasn't even that there were only a few people on the road; the street was entirely deserted, almost like something you'd expect from a ghost town.

A sound like an explosion suddenly erupted from an alleyway to his left, causing Touma to whirl around in surprise. A foreign-looking silver-haired girl in some kind of white robes dashed breathlessly out of the alley. She gave a yelp as she narrowly avoiding crashing into Touma.

"Don't just stand there, run!" she yelled over her shoulder as she continued fleeing down the street.

"Run? Wha-? Hey, what's going on?" he asked. He made to follow after the girl, but was interrupted by a shockwave of sound as a woman with long black hair tied in a ponytail came rushing out of the alley at what must have been supersonic speeds and charged straight into the girl. She went flying, crashing heavily into the building on the opposite side of the street, the impact actually leaving cracks in the concrete wall.

Touma was too stunned at what had just happened to move. That speed and impossible strength was something only someone with the power of a Level 5 could manage, but he was pretty sure none of the city's Level 5's had any kind of abilities like that. Besides, the woman – who was carrying a katana over two meters long and was dressed in a white shirt tied above her navel and jeans with one leg completely cut off – certainly didn't look like she belonged in Academy City.

She was standing impassively over the motionless body of the girl, her attention entirely focused on examining the girl.

"H… hey," Touma called out nervously.

The woman turned, acknowledging Touma's presence for the first time. "What are you doing here, boy?" she asked, her face frozen in a neutral expression.

"Shouldn't I be the one asking you that?" he shot back, somehow finding more confidence now that he was talking. "Why were you chasing her?"

The woman ignored his question, shifting her hand to the hilt of her sword. "I'd prefer not to have to hurt you, boy. Now answer me: How did you get here?"

"You'd rather not hurt me, but you were perfectly fine with killing that girl?"

"Hmm? Oh. She's perfectly fine. I would have had to use my full strength to even have a chance of breaking through the Walking Church she wears," she said, glancing back at the girl. "Now, you are obviously not a magician. So how did you get past Stiyl's runes?"

"Magician? What the hell is that supposed to mean?"

"Exactly what it sounds like: someone who uses magic. Are you an idiot or something?" a new voice contemptuously answered from behind him.

Touma whirled around. An abnormally tall boy with brilliant crimson hair and sporting a barcode underneath his right eye stood before him, smoking a cigarette and dressed in some sort of priest's robes.

"Kanzaki, take Index and get out of here. I'll handle this," he spoke, addressing the woman.

"Shouldn't we deal with this boy first, Stiyl?"she asked.

"Getting her out of here is more important. I doubt some random high school student will give me much trouble."

Kanzaki nodded, bent down, and effortlessly lifted the girl over her shoulder.

"Hey, put her down!" Touma yelled as he started to move towards her, his body tensed for a fight.

Whatever the woman did, it happened too fast for Touma to even make out. Her arm seemed to blur with impossible speed, and then the asphalt erupted directly in front of him. He threw his arms up over his face as the shrapnel slammed into his body, knocking him onto his back. By the time he was able to shakily force himself back onto his feet, she had disappeared – along with the girl.

"It was no use trying to catch her anyway. That woman is a Saint, although you probably wouldn't have any idea what that means." said the red-haired boy as he took a lazy drag on his cigarette. "Now, it would probably be better for you if you just forgot that any of this ever happened. You have no reason to get involved with this."

"_Forget_ about this? Like hell that's happening. How can I do that when I just saw you people brutally attack some girl right in front of me?" said Touma while massaging his bruised arms. "Where are you taking her?"

"I told you, this is nothing that concerns you. You shouldn't have even been able to be here in the first place. Which reminds me, how did you manage to get past my runes anyway?"

"You… Stop talking nonsense and tell me what you did with her!" Touma roared as he charged at the priest, his right fist swinging through the air.

Stiyl dodged backwards and flung some kind of playing card at the ground at Touma's feet. Touma barely had time to register the peculiar symbol drawn on the card before it exploded in flame, the blast throwing Touma to the ground.

The priest took his cigarette out of his mouth and flicked it off to the side, leaving a glowing trail of sparks in the air. The sparks glowed brightly and burst into flame, the blaze taking the shape of a sword in Stiyl's hand. "You know, your life would have been a whole lot less painful if you just knew when to walk away," said Stiyl as he slashed at Touma. The flame sword in his hand extended and turned into a fireball which engulfed Touma.

But with the sound of shattering glass, the fireball dissipated into nothingness. "What?" Stiyl gasped. The boy was standing with his right arm stretched out in front of him, completely unscathed.

Stiyl conjured another sword and swung it at Touma, the flames leaping through the air towards the boy as if they had a life of their own. Touma dashed straight toward them, right hand still held out in front of him like a shield. He effortlessly batted aside the flames, which again disappeared as if Stiyl had never cast them.

Touma rapidly closed the remaining distance between him and the priest, who was apparently too shocked to move out of the way. He pulled his fist back and swung at Stiyl, putting all his momentum and weight into the blow.

Touma stumbled awkwardly as he encountered only empty air. His hand passed straight through the image of Stiyl in front of him, which vanished with the sound of breaking glass.

As the mirage was destroyed, the priest reappeared on Touma's left. He conjured another sword and lashed out at Touma, who twisted awkwardly to try to defend against the attack. He overbalanced and fell heavily to the ground, and he felt a searing heat pass over him as the fiery blade sliced through the air above him.

Touma rolled onto his back only to see the priest standing over him, already swinging the sword down towards him. He stuck his hand out to block and kicked wildly at the priest's leg. His foot connected with Stiyl's knee, and the priest gave a grunt of pain as his leg collapsed. Touma scrambled to his knees and lunged at Stiyl, his fist slamming into his face and knocking him to the ground, unconscious.

* * *

><p>Touma stood over the body of the red-haired priest, unsure of what to do. Calling Anti-Skill was probably the right thing to do, but if he did that he probably wouldn't ever get any answers as to why these people were going around trying to abduct some girl. The best way to find out what was going on would be to just force it out of the priest, but he wasn't sure how to go about waking up an unconscious person. Although he'd been in plenty of street fights with Espers before now, he'd never really needed to hang around to ask questions afterward.<p>

The priest gave out a low groan and stirred, saving Touma from having to figure out at least that part of the situation. Touma reached down and pulled the priest up by the collar of his robes.

"Do you feel like answering me now?" he growled. "Where have you taken her? Why were you attacking her?"

"We… we're not going to hurt her," Stiyl muttered, still groggy from the blow and holding the side of his jaw in his hand. "We're trying to… save her life."

"What? You expect me to believe that? It sure looked like you were trying to kill her!" shouted Touma.

"We had no choice but to attack her. She doesn't remember us, and there's no way she would trust us if we approached her."

"Doesn't remember? That's hardly a believable explanation."

"I don't expect you to believe any of this. I told you, this is none of your business and you'll be far better off if you don't ask questions."

Touma glared determinedly at the priest, still holding tightly onto his robes. "You say you're trying to save her life, but there's absolutely no way I'm going to let you go just because of that. To me, it looks exactly like you were trying to kill her. If you want me to walk away from this, you're going to have to try real hard to convince me."

Stiyl was silent for a moment, his face averted from the boy's, before he started speaking again. "That girl has an absolutely perfect memory. Every detail she has ever seen, every conversation she has ever had, gets recorded in her memory. And she cannot forget any of it. But 85% of her memory is already taken up by a massive library of books and scrolls. She can only remember a year's worth of other memories before her brain starts to break down. That's why we need to capture her. We have to wipe her memory before that happens, and there's only about a week left until then."

"Why the hell don't you just get rid of those books, then? How can you possibly justify placing more importance on those books than her-"

"Do you think I haven't thought of that already?" Stiyl furiously interrupted. "I tried to remove them as soon as I learned what was happening. But those books are… special. It's impossible to remove them."

"If you can remove the rest of her memories, how can these books be impossible to remove? Look, Academy City has some of the best neurologists on the planet. And one of the Level 5 Espers is a telepath. If there's any place where she can get help, it's this city. Besides, that '85%' number seems really fishy to me. Even someone who knows almost nothing about the brain would be able to tell you that memory doesn't work like that."

"This city can't help her. This is a problem that is outside the realm of your science."

"Huh? Outside the realm of science? What the hell are you talking about?"

"Look, this really isn't something you want to know." Stiyl glanced at Touma's determined expression and sighed before continuing. "Fine, if it's the only way to convince you... I'm talking about magic."

"Magic? Don't lie to me, dammit. I want to know what's going on, not some nonsense you've made up!"

"It's hardly nonsense. Don't you think it's strange that I'm not a student of this city, and yet I can conjure fire from nothing? Nowhere else on the planet has anything remotely close to the research or experiments in Academy City. And if I'm not an Esper, what other explanation can there be?"

"I… I don't know," said Touma, "But I can't just believe something as ridiculous as that. I'm pretty sure some people became Espers through natural means."

"I never expected you to believe it, remember?" said Stiyl as he pulled himself out of Touma's grip and forced himself to his feet. "But what you believe doesn't change reality. Magic is real, just as real as your science is."

"Even if you're telling the truth, why do you have to attack her? Can't you just explain what's going on to her?"

Stiyl sighed and pulled a cigarette packet out from under his robes. "She wouldn't trust us. She knows the value of the books she has. She would assume that we were trying to recover their information, especially if we told her we were going to do things to her mind." He lit a cigarette with a small flame conjured from his finger.

Stiyl turned away from Touma. "So, this is just the way things have to be. We'll wipe her memory, and she'll be able to live another year. That's all there is to it," Stiyl said as he placed the cigarette in his mouth.

"…Like hell I'm going to just accept that. I'm not going to let you do this to her again. If you really want to save her, why put her through this? Why not stay with her after wiping her memories? That way, you don't have to hunt her down or anything. You have no right to be attacking her when you could have been going about this a better way!"

Stiyl stopped in his tracks, his shoulders trembling. "Shut up, boy…"

"You said there's a week left, right? I'm sure there would be some neurologists in this city that would be willing to help. You won't know that they won't be able to help if you haven't tried it. I'm not going to just give up on her until everything possible has been tried."

Stiyl whirled around, his expression contorted with fury. "I said shut UP!" he snarled, advancing on Touma. "You have NO idea what you're talking about! You know nothing about her. You didn't even know she existed 15 minutes ago! What gives you the right to talk about saving her, or to lecture me about a situation you can't possibly understand? Do you have any idea how long I've been doing this? How many years it's been? How many times I've watched her die right in front of me? How many times I've been forced to destroy her mind so that her body would stay alive another year? Don't you DARE lecture me on 'giving up.'"

Touma was taken aback. The anguish in the priest's words was genuine. No actor could have faked that. Against his better judgment, and against everything he know about how the world worked, Touma found himself starting to believe that Stiyl was telling the truth.

"And I haven't given up," Stiyl said as he turned to walk away. "I swore with every part of my soul that I would do everything I could to fix this, no matter the cost. But until then, I have no other choice. I have to keep doing this, keep erasing her memory year after year, in order to buy time until I can figure out how to save her."

"Anyway, this doesn't concern you, and even if it did it's entirely out of your hands. Kanzaki will be well outside the city by now. In a few hours, Index will be back in England, and in a week's time, well… she won't remember anything."

Stiyl vanished into an alleyway. Eventually people began to filter back onto the street, a crowd forming as they stopped to stare at the devastation caused to the street, and the teenage boy standing motionless in the middle of it all.

* * *

><p><strong>Author's Note:<strong> This took me longer than I would have liked to get it to the point where I was happy with it. Sorry for the delay.

So, Touma completely failing to save Index is going to have some wide-ranging consequences. The most immediate of which are that Touma won't lose his memories, and he won't get involved in quite a bit of the earlier magic stuff.

Speaking of which, I did originally want to do something about how Stiyl handled things without Touma around, but I realized it was kind of bogging down the story. This is supposed to be more about the scientific shenanigans after all. So, we're not going to be covering the Deep Blood arc, and I see no good way to get Touma involved in the Railgun arcs. Therefore, he gets a nice, relaxing, unlucky month before getting involved in anything again.

The fight with Stiyl seemed kind of short, but then again he never got a chance to get Innocentius set up. Later action scenes should be more fulfilling than this one.

Anyway, thanks for reading! Hope you're enjoying it.


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